Modern day music is laden with the sounds of various instruments which are electronically amplified and modified in myriad ways to achieve different musical sound effects. One common type of sound effect often used in conjunction with electric guitars and the like is achieved through the introduction of distortion to the electric audio signals produced by the instrument prior to or during final amplification of the signal before conversion to sound. Units which perform these sound modification functions are well known and are commonly referred to as "fuzz boxes".
A well known technique used in such "fuzz boxes" is to introduce additional harmonic content to an audio signal by clipping. Two aspects of clipping are important in establishing the quality and type of additional harmonic content that is obtained: abruptness of clipping and symmetry of clipping. The symmetry of clipping, i.e. the extent to which the positive and negative polarity portions of the audio signal are clipped by the same amount establishes whether the harmonics are odd numbered harmonics or even numbered harmonics. The abruptness of clipping, i.e. whether the signal is severely clipped (hard clipping) or gradually reduced or compressed (soft clipping) determines whether most of the resulting harmonics are high order harmonics or low order harmonics. A graphic representation of these two clipping parameters is shown in FIGS. 2b-2c.
FIG. 2a is an illustration of an undistorted audio input signal for comparison with the waveforms of FIGS. 2b-2e which are illustrations of the waveforms resulting from clipping the undistorted waveform of FIG. 2a in different ways. FIG. 2b illustrates the waveform resulting from "soft" clipping of the signal of FIG. 2a. As will be explained in more detail hereinafter, the distortion circuit 12 performs a soft clipping operation on the signal by slightly overloading an input stage of an amplifier amplifying the undistorted signal. As seen in FIG. 2a, this type of clipping results in the signal to be undistorted within an amplitude range A less than the peak amplitude of the undistorted signal and to be gradually reduced with respect to the undistorted signal beyond this range. Soft clipping results in the development of low order harmonics which results in a sound that is subjectively characterized as being a "mellow" or "warmer" sound. This type of distortion is sometimes referred to as soft distortion. More specifically, the type of distortion introduced by soft clipping generally gives rise to a harmonic series in which the amplitude of a given harmonic dies off faster than the square of the harmonic number. In this type of distortion, there is very little energy, approximately only one or two percent, in harmonics above the fifth harmonic. Alternately, if the energy in the higher order harmonic is more than a few percent of the total sound energy and these higher order harmonics lie in the frequency band between 1 kHz and 5 kHz, the frequency range in which hearing is most sensitive, then the resulting sound has a harsh, strident quality.
Illustrated in FIG. 2c, is the output signal developed from performing a hard clipping operation on the undistorted signal of FIG. 2a. In this type of distortion, the waveform above a preselected amplitude, amplitude C, for instance, is abruptly terminated. The distortion resulting from hard clipping results in a harmonic series that generally dies off in inverse proportion to the harmonic number as is sometimes referred to as hard distortion. For instance, the tenth harmonic will be one half as strong as the fifth harmonic. In a waveform which is severely hard clipped, such that it almost appears as a square wave, the energy in the overtones is greater than the energy in the fundamental frequency, and most of this energy resides in harmonics above the fifth harmonic. The sound produced from a hard clipped audio signal is sometimes subjectively characterized as a "raspy" or "dirty" sound.
FIG. 2d illustrates the output waveform produced from symmetrically clipping the undistorted input waveform of FIG. 2a. FIG. 2e illustrates the output waveform developed from asymmetrically clipping the undistorted input of FIG. 2a. As stated, the clipping symmetry determines the percentage of even harmonics that are present in the distorted waveform. Completely symmetrical clipping, as in FIG. 2d, in which both the positive and negative portions of the waveform are clipped by equal amounts, results in the generation of only odd harmonics. This type of distortion is thus referred to as odd distortion. The sound produced from a signal containing only odd harmonics is sometimes subjectively characterized as having a "hollow" or "woody" quality. The waveform developed by asymmetrically clipping in which the positive portion of the waveform is clipped by a greater or lesser amount than the negative portion of the waveform, as illustrated in FIG. 2e, results in a signal containing even harmonics. If the clipping is completely asymmetrical, then only even harmonics are produced which are octaves of the fundamental frequency and the odd harmonics thereof. This type of distortion is thus referred to as even distortion. Some musicians subjectively characterize the sound developed in response to a signal containing only even harmonics as more "musical".
Heretofore, the distortion characteristics obtained by soft clipping has only been achieved through the use of vacuum tube amplifiers. The use of semiconductors is generally preferred over vacuum tube amplifiers due to the larger size and power requirements of vacuum tubes over semiconductor amplifiers and because of the generally better reliability of semiconductor amplifiers. Thus, while this type of soft clipping distortion can be achieved through the use of vacuum tube amplifiers, because of the reasons noted above and because vacuum tube circuits are not generally compatible with semiconductor circuits which are required for achieving other types of distortion, there is a need for a semiconductor circuit that will achieve these same results.
A common problem with known "fuzz boxes" in which distortion is obtained by clipping arises from the fact that electric guitars or other instruments which produce the input audio signal to the "fuzz boxes" are often provided with a volume control to enable the musician to make level changes to the audio signal. If the clipping level is not changed in the same sense as the input level, changing the input level can result in undesired changes in the type of distortion produced by the "fuzz box". For example, if the clipping level is set to obtain soft clipping type distortion, and the input audio signal amplitude is doubled, hard clipping may result unless the clipping level is proportionately raised. This input gain sensitivity which precludes or limits the musician from maintaining a selected type of distortion while making volume changes at the instrument has therefore been an undesirable feature of known distortion circuits.
A cost disadvantage of known distortion circuits having a multiple distortion capability is that separate circuits have been required for each of the different types of distortion or the circuits could provide only one type of distortion at a time.